Extreme ultraviolet (EUV) radiation \(E>13.6\)  eV) from massive stars ionize hydrogen atoms and create an H ii region in their environment. This ionized gas is separated from the surrounding nascent molecular cloud by a photodissociation region (PDR) where penetrating FUV photons dissociate molecules and ionize low ionization potential atoms. These FUV photons ultimately heat the PDR gas to temperatures of 200-1000 K. The neutral gas (i.e., hydrogen in neutral form) couples to the FUV photon field through the photoelectric effect on large polycyclic aromatic hydrocarbon molecules (PAHs) and very small grains [1]}. This same process heats the gas in diffuse interstellar clouds [2]}. Dense PDRs in star-forming regions are bright in the far-infrared (FIR) atomic cooling lines, e.g. the [C ii] \(158\,\mu \mathrm {m}\)  line at moderate densities and warmish temperatures [3]}, [4]}, as well as the PAH emission features [5]}, fluorescent ro-vibrational lines [6]}, [7]}, [8]}, pure rotational H\(_2\)  lines [9]}, [10]}, [11]}, warm dust [12]}, [13]}, and a plethora of molecular radicals and reactive ions [14]}, [15]}, [16]}, [17]}. Their high surface brightness and compact size make dense PDRs very appropriate targets for observational studies on the interaction of massive stars with their interstellar environment. This allows detailed studies of the physics and chemistry relevant to radiative feedback by massive stars. In particular, observations of the dominant atomic fine structure lines, through the energy balance, provide a direct probe of the heating processes of interstellar neutral atomic gas [5]}, [19]}, [20]}, [21]}.
